There are various types of digital communications systems for establishing communications between digital user devices, such as computers and workstations. For example, a system designed to interconnect computers over a restricted geographical area (generally up to about one mile) is known as local area network (LAN). One example of such as system is Ethernet, which was designed by Xerox during the late 1970's. This operates at 10 Mbs and the data are sent over twisted pairs in the form of Ethernet frames.
For greater distances, wide area networks (WANs) are employed. One form of WAN employs ATM (Asynchronous Transfer Mode). ATM employs 53 byte cells as a basic unit of transfer. Each ATM cell is divided into 5 bytes of ATM layer overhead and 48 bytes of ATM payload. An ATM network is essentially statistical in nature with the ATM cells being transmitted over virtual channels established within the network.
Through the use of a cell switching multiplexing scheme, ATM exploits bandwidth utilization by taking advantage of the statistical multiplexing benefits of a switching fabric. In a homogeneous ATM environment, ATM terminal adapters are connected to the ATM switching fabric, forming a potentially fully meshed logically connected communications infrastructure. An ATM adapter is any edge device which interfaces the ATM cell stream to an ATM services consumer. Typical examples of ATM adapters include ATM network interface cards (NICs) for digital computers.
Generally, networked computers are provided with LAN adapters for connection to a local area network, such as Ethernet. Such LAN adapters do not permit them to be connected to Wide Area Networks, such as ATM networks. While it is possible to fit special ATM adapter cards into networked computers, this requires physically accessing the computers and supplying appropriate driver software.
PCT application WO/93/26107 discloses an ATM-Ethernet portal, which assembles ATM cells into Ethernet frames and vice versa so as to enable devices attached to remote Ethernet LANs to communicate transparently over an ATM network. A portal located between each Ethernet segment and the ATM network segments outgoing Ethernet frames into ATM cells and vice versa. The portals operate at a low level in the ISO model and are not capable of communicating with non-Ethernet devices. Protocols located above the data link layer (layer 2) are not interpreted by the portals.
In one embodiment, the Ethernet frames are sent from one Ethernet to another over a virtual circuit configured by hand. This is an inefficient and time-consuming process. In another embodiment, permanent virtual circuits are established through the ATM network between each pair of portals, and a transmitting portal sends cells to each portal. This is an inefficient use of bandwidth since cells must be sent to all the portals, needlessly clogging up the network.
In another embodiment, a Connection Processor attached to a host on one of the Ethernet segments sends out instructions to configure the multipoint connections between the portals attached to the various Ethernet segments. The transmitting portal then sends outgoing ATM cells to all portals on the multi-point connection (inefficient use of bandwidth). In another embodiment, a primitive form of routing is employed in that the Connection processor advises the local portal which one of the pre-established virtual circuits to send the cells out on. This only works if the connection processor attached to the local portal has already set up a PVC to that remote portal.
In PCT application WO/93/26107, permananent virtual circuits are required between each pair of portals, which severely limits the size of the system. This is analogous to an unswitched telephone system, where each pair of telephones must be connected to each other. The number of connections increases exponentially with the number of telephones. Furthermore, each portal must know which circuit leads to the portal to which the destination device is connected, so each look-up table in each portal must be updated each time a device is attached to the network. The system described in the above application is intended only for a small number of portals. If no circuit exists to a portal, the portals cannot communicate over the network.
Finally, the system described in the above PCT application is merely designed to transport Ethernet frames over an ATM fabric between Ethernet segments. It does not permit communication between an Ethernet-attached device and an ATM-attached device.
EP 473,066 describes a network wherein LANs are connected to an ATM network via respective bridges. Each bridge maintains the addresses of all the devices on the network. The problem with this arrangement is that it is not responsive to the presence of new terminals, so if a bridge does not know the address of a destination terminal, it has to flood the entire ATM network, thus undesirably consuming large amounts of bandwidth.
EP 524, 316 also describes a system for interconnecting LANs (connectionless networks) through ATM fabrics (connection-oriented networks). In this system, connectionless servers are associated with each ATM switch, and segmented LAN frames must first be routed as ATM cells through a permanent virtual connection (PVC) to a sending server and then from the sending server through the ATM fabric to the receiving server, from where the cells are sent over a second permanent virtual connection to a reassembler. While this is an improvement described in the prior art to this patent, this system requires a connectionless server to be associated with each switch and and PVC (which is expensive) to be maintained between the connectionless server and terminal it serves. Furthermore, each connectionless server has to maintain the numbers of the destination terminals. If the connectionless serve does not know the destination address, it cannot forward the cells to the correct destination without flooding the network, an operation which consumes large amounts of useless bandwidth.
All the prior art systems are designed as a means of establishing communication between devices attached to LANs interconnected through an ATM network. None of these systems contemplates the possibility of LAN-attached devices communicating with other devices directly connected to the ATM network.
An object of the invention is to alleviate these disadvantages.